Megacities around the world face air quality problems due to exhaust emissions from large vehicle fleets. Chemical reactions involving volatile organic compounds (VOC) from automobile exhaust are known to produce ground level ozone which poses a threat to human health and plant life. Additionally, some of these organic compounds are known carcinogens. While the dangers of VOC are well known, their production from in use vehicles and aircraft are relatively poorly understood. This dissertation will describe how a Proton Transfer Reaction Mass Spectrometer (PTR-MS) was altered to make the first on-road, speciated VOC measurements of its kind and how the response of the instrument in such a complex sample matrix was interpreted. The developed experimental technique was then used to observe aircraft exhaust plumes from in-use commercial aircraft, and to further characterize aircraft exhaust in a ground based experiment. PTR-MS was an appropriate technique for these measurements due to its ability to perform rapid, quantitative measurements. A commercial PTR-MS instrument was modified to operate in a mobile laboratory with capabilities of sampling and analyzing intercepted air in real-time. Automobile and aircraft exhaust are both complex mixtures of VOC which produce complicated ion responses in the PTR-MS. Accurate quantification of a number of important VOC is demonstrated through a series of thorough intercomparison studies to other measurement techniques such as Gas Chromotography Flame Ionization Detection (GC-FID), Solid Phase Microextraction (SPME) Gas Chromatography Mass Spectrometry (GCMS) and Differential Optical Absorption Spectroscopy (DOAS). A calibration method was developed to increase quantification accuracy for some important compounds observed in exhaust in this study. The interpretation of the PTR-MS response to each exhaust type is now well understood. Once the PTR-MS response was validated by comparison to other techniques, the results from the measurement campaigns provided the basis of several papers extending the understanding of exhaust compositions. Among these results, emission inventories of several compounds were determined for various classes of vehicles and for aircraft. These emission inventories can be used in models to predict the impact of these emissions on local air quality. Additional results from these studies are also presented in this dissertation.Holton, J. R.; Haynes, P. H.; McIntyre, M. E.; Douglass, A. R.; Rood, R. B.; Pfister,
L. Stratosphere-Troposphere exchange. Reviews of Geophysics 1995, 33 (4),
403-439. ... 2000, 34 (27), 4611-4619. 19. Whitfield, J. K.; Harris, D. B.
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